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Prof. J. J. Thomson. 



[Mar. 9 ? 



charge passes through gases, and seem to have special reference to a 

 view which I have long held, that the discharge through gases is 

 accompanied by chemical changes analogous to those which take place 

 in electrolytes conveying currents, I was anxious to repeat and, if 

 possible, extend them. On attempting to do this, I met with very 

 considerable difficulties, and it has taken more than a year's work to 

 overcome these, and to arrange the experiments so as to get definite 

 and consistent results. For this reason, as well as from the fact that 

 my results differ very materially from those obtained by previous ex- 

 perimenters, I shall enter at greater length into the details of the 

 experiments than would otherwise be necessary. 



The form of apparatus which. I now use, though similar in its main 

 features to that used by Perrot, differs from it in some respects. 

 Before describing the apparatus in detail, I will indicate the chief 

 points of difference between it and Perrot's. 



One source of doubt in Perrot's experiment seemed to me to arise 

 from the proximity of the. tubes surrounding the electrodes to the 

 surface of the water. These tubes were narrow, and, if they got 

 damp, the sparks, instead of passing directly through the steam, 

 might conceivably have run from one platinum electrode to the film 

 of moisture on the adjacent tube, then through the steam to the film 

 of moisture on the other tube, and thence to the other electrode. If 

 anything of this kind happened, it might be urged that, since the 

 discharge passed through water in its passage from one terminal to 

 the other, some of the gases collected in the tubes might have been 

 due to the decomposition of the water and not to that of the steam. 

 To overcome this objection, I have (1) removed the terminals to a 

 very much greater distance from the surface of the water, and placed 

 them in a region surrounded by a ring burner, by means of which the 

 steam can be heated to a temperature of 140° or 150° C. ; (2) I have 

 got rid of the narrow tubes surrounding the electrodes altogether by 

 making the tubes through which the steam escapes partly of metal, 

 and using the metallic parts of these tubes as the electrodes. 



Thoagh I prefer this method of arranging the electrodes as being 

 somewhat more convenient than Perrot's form of the experiment, in 

 which the electrodes were wires surrounded by glass tubes, I have 

 repeated the experiments described below, using wire electrodes ; the 

 results, however, were precisely the same as those obtained when the 

 tubular electrodes were used. One great advantage of these tubular 

 electrodes is that the quantity of metal in them is large enough to 

 keep them quite cool during the discharge ; while, when wire elec- 

 trodes are used, the end of the negative terminal becomes red hot if 

 any considerable current passes through the steam. 



Instead of following Perrot's plan of removing the mixed gases 

 from the collecting tubes e, e, fig. 1, and then exploding them in a 



